3.19 Concentration Cell Corrosion

Question 1

Concentration cell corrosion is not a separate corrosion mechanism but has an influence on a number of commonly occurring corrosion phenomena, including :

Answer 1

1. under-deposit corrosion,
2. crevice corrosion,
3. contact point corrosion (also called touch point corrosion),
4. soil/air interface corrosion

Question 2

Concentration cell corrosion occurs in aqueous environments. The occluded cell ocuors commonly in areas like:

Answer 2

1. beneath the deposit,
2. inside the crevice,
3. where the pipe sits on its support,
4. the first few inches of soil beneath the ground surface,
5. where pipe is contacted by wet insulation

Question 3

In most cases, the difference between the environment in the occluded cell and the area around it is the oxygen concentration. Because the the area outside the occluded cell has a __________ oxygen concentration than that within the occluded cell. This differential oxygen concentration cell, causes the _________oxygen area to corrode preferentially to the higher-oxygen area.

Answer 3

higher ; lower

Question 4

Concentracion cell is essentially a galvanic couple where the ___________ area is the anode and the ____________ area is the cathode.

Answer 4

low-oxygen higher-oxygen

Question 5

Concentration cell corrosion can also occur in the absence of air or oxygen, such as under deposits. In this case, due to

Answer 5

othes chemical species as perhaps sulfur or chlorides.

Question 6

Crevice corrosion in 300 series SS and similar alloys is related to the __________ content of the environment as well as the pH and temperature. It is closely related to the _______________ in these materials

Answer 6

chloride ; chloride pitting phenomenon

Question 7

process where the pH within the crevice decreases as the corrosion proceeds, accelerating the corrosion rate.

Answer 7

Metal ion hydrolysis

Question 8

Resistance to chloride crevice
corrosion (and chloride pitting) of stainless steels is related to the amount of _______________, as well as certain other chemical additions, in the alloy. It is typically represented by the __________

Answer 8

Cr and Mo ; PREN

Question 9

concentration cell corrosion will typically result in an unevenly corroded surface often with ____________. Crevice corrosion in austenitic stainless steels due to chlorides is typically an accumulation of ____________________

Answer 9

pitting ; sharp, narrow, deep pits